Measurement of earthquake ground shaking at characteristic sites in Christchurch

Abstract

The relative ground response due to microearthquakes has been examined at six sites in Christchurch. The five recorded sites in central Christchurch all experienced considerably greater shaking than a rock site in the Cashmere Hills. This result is in general agreement with recent modelling studies that predicted greater shaking in central Christchurch. The greatest shaking was recorded at Christchurch Womens Hospital, where the response spectra was 10-20 times that value of the rock site spectra in the period range 0.3 to 0.6 sec. a site at Latimer Square produced ratios of 4 to 10 times the rock site at short periods while three other sites showed 2-4 times the shaking of the rock site.

The sites were chosen to sample the range of soil types and depths in the central city. A total of 12 earthquakes were used in the study. The earthquakes ranged in magnitude from 3.0 to 5.7 and occurred between 25 and 700 km from the centre of the seismograph array. The results are presented as velocity seismograms at each site and as averaged ratios of Fourier and response spectra of the seismograms compared with the site bedrock.

There is a wide scatter about the mean for all sites. Ratios for individual earthquakes can range from 0.5 to 1.5 times the mean ratio. Thus relative amplifications can vary significantly for different earthquakes.

Results from other studies suggest that the frequency of amplified shaking often remains the same for large damaging earthquakes. The amount of amplification for large ground motions is dependent on the properties of the underlying geologic materials. Some soft soil sites will experience less amplification during large motions, however greater damage is still expected at the soft soil sites relative to the firm sites for earthquakes occurring outside of Christchurch.